1. Field of the Invention
The present invention relates to a membrane for liquid mixture separation to be used in the pervaporation or vapor permeation.
2. Description of the Related Art
Over the past years, a great deal of research has been devoted to separating liquid mixture using the pervaporation or vapor permeation technique through non-porous polymeric membranes. This separation process can be applied successfully to mixtures that are difficult to separate by is the case conventional techniques, such as is the case for azeotropic mixtures or mixtures of liquids with very small differences in vapor pressures. For example, studied the separation of an azeotropic mixture through polyvinyl alcohol membrane was studied. (U.S. Pat. No. 2,953,502) Recently, the separation of water-formaldehyde mixture thorugh styrene-acrylic acid copolymer membrane has been reported. (U.S. Pat. No. 4,035,291). These membranes, however, have not been used practically because of insufficient permeation rates or insufficient separating factors.
Under these circumstances, the pervaporation and vapor permeation are the more recently recognized separation processes. They are regarded as a promising separation process which will supplement or supersede the distillation. It is considered that they are especially suitable for the separation of water-ethanol azeotrope. Therefore, active research and development works have been performed on these methods and some practical membranes for them have been proposed. For example, a cellulos membrane and a polyvinyl alcohol membrane are disclosed in Japanese Patent Laid-open No. 109204/1984 and a cross-linked polyethyleneimine membrane is disclosed in Japanese Patent Laid-open No. 55305/1984. However, these membranes disclosed so far are not of practical use because of insufficient separating factors or permeating rates to similar physical propaties, as in the case of water and methanol. In addition, they show insufficient resistance to organic solvents such as DMF and (N,N-dimethylformamide). These shortcomings restrict their use to the separation of water from limited substances, such as alcohols and ketones having three or more carbon atoms.